65-node simple mesh test (and counting... ;-)

C. Scott Ananian cscott at laptop.org
Fri May 9 18:42:45 EDT 2008

On Fri, May 9, 2008 at 6:04 PM, Polychronis Ypodimatopoulos
<ypod at mit.edu> wrote:
>> On a wireless network, broadcasts are successfully received with much
>> lower probability.  RF is mysterious and magical, and all sorts of
>> connection asymmetries, near-field effects, and radiation lobe
>> patterns conspire to make it unlikely that *everyone* can hear you
>> equally at once -- and then you get into remote collisions and other
>> mechanisms that make you unaware that not everyone heard you.  And
>> there is not 'ack' mechanism for 802.11 broadcast.
> All these are true also, but I think we're mystifying things a little bit
> here. The wireless medium is unpredictable mainly because its properties are
> also a function of time (a non-issue in wired networks), but at least (thank
> God!) it [the wireless medium] does not discriminate between broadcast and
> unicast frames! Adding an ack scheme to broadcasts should yield equal (or
> even better due to lowered speed) reliability using broadcast frames. Even
> without the ack scheme, I noticed that, on average, some 95% of the data
> transmitted over broadcast are successfully received on all nodes. We are
> throwing this away by discarding it on our wireless interfaces.

I think the real key here is that Cerebro is attempting to maintain a
full enumeration of all the nodes on the local network.  This allows
it to efficiently collect ACKs, since it knows all of the nodes who it
should receive an ACK from.  In "standard" ethernet, no one is
expected to maintain this list of nodes, and so ensuring you've got an
ACK from every receiver is "impossible".

 ( http://cscott.net/ )

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